U.S. patent application number 12/678949 was filed with the patent office on 2010-08-19 for method for receiving preamble from other communication system and method for adaptively changing the measurement gap to discover the other communication system.
Invention is credited to Yong Ho Kim, Tae Gon Kong, Chang Hun Lee, Jin Lee, Gi Won Park.
Application Number | 20100208674 12/678949 |
Document ID | / |
Family ID | 40468600 |
Filed Date | 2010-08-19 |
United States Patent
Application |
20100208674 |
Kind Code |
A1 |
Lee; Jin ; et al. |
August 19, 2010 |
METHOD FOR RECEIVING PREAMBLE FROM OTHER COMMUNICATION SYSTEM AND
METHOD FOR ADAPTIVELY CHANGING THE MEASUREMENT GAP TO DISCOVER THE
OTHER COMMUNICATION SYSTEM
Abstract
A method for receiving a preamble signal of another
communication system and a method for adaptively changing a
measurement gap to discover another communication system are
disclosed. A method for receiving a preamble signal from a base
station of a second communication system by a communicating mobile
station in a first communication system includes receiving
measurement gap scheduling information from a base station of the
first communication system, if a preamble signal of the second
communication system is not received during an assigned measurement
gap, changing the measurement gap, and receiving the preamble
signal of the second communication system during the changed
measurement gap.
Inventors: |
Lee; Jin; (Gyeonggi-do,
KR) ; Kim; Yong Ho; (Gyeonggi-do, KR) ; Kong;
Tae Gon; (Gyeonggi-do, KR) ; Park; Gi Won;
(Gyeonggi-do, KR) ; Lee; Chang Hun; (Gyeonggi-do,
KR) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
40468600 |
Appl. No.: |
12/678949 |
Filed: |
September 18, 2008 |
PCT Filed: |
September 18, 2008 |
PCT NO: |
PCT/KR2008/005513 |
371 Date: |
March 18, 2010 |
Current U.S.
Class: |
370/329 |
Current CPC
Class: |
H04W 36/14 20130101;
H04W 36/0088 20130101 |
Class at
Publication: |
370/329 |
International
Class: |
H04W 72/12 20090101
H04W072/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 21, 2007 |
KR |
10-2007-0097042 |
Claims
1. A method for receiving a preamble signal from a base station of
a second communication system by a communicating mobile station in
a first communication system, comprising: receiving measurement gap
scheduling information from a base station of the first
communication system; if a preamble signal of the second
communication system is not received during an assigned measurement
gap, changing the measurement gap; and receiving the preamble
signal of the second communication system during the changed
measurement gap.
2. The method according to claim 1, wherein no signal is
transmitted and received to and from the base station of the first
communication system during the measurement gap.
3. The method according to claim 1, wherein the measurement gap
scheduling information includes a measurement gap size and a
measurement gap assignment element.
4. The method according to claim 3, wherein the changing the
measurement gap changes at least one of the measurement gap size
and the measurement gap assignment element.
5. The method according to claim 3, wherein the measurement gap
assignment element includes at least one of a measurement gap
interval, a measurement window, and the number of times measurement
is performed within the measurement window.
6. The method according to claim 1, wherein the measurement gap
scheduling information is index information based on a measurement
gap pattern table in which measurement gap scheduling parameter
values are set.
7. The method according to claim 1, wherein the changed measurement
gap is re-changed to correspond to a preamble transmission period
of the second communication system after the preamble signal of the
second communication system is received.
8. The method according to claim 7, further comprising:
transmitting base station identification information obtained from
the preamble signal of the second communication system to the base
station of the first communication system; and receiving system
information for the second communication system from the base
station of the first communication system.
9. The method according to claim 1, wherein the mobile station
further receives the preamble signal of the second communication
system at least once to obtain channel quality information for the
second communication system.
10. The method according to claim 9, wherein if a value of the
obtained channel quality information for the second communication
system is above a prescribed reference, the mobile station performs
handover to the base station of the second communication
system.
11. A method for receiving a preamble from a base station of a
second communication system by a communicating mobile station in a
first communication system, comprising: if a preamble signal of the
second communication system is not received during a measurement
gap assigned from a base station of the first communication system,
adaptively changing the measurement gap; receiving the preamble
signal of the second communication system during the changed
measurement gap and re-changing the changed measurement gap to
correspond to a transmission period of the preamble signal; and
further receiving the preamble signal of the second communication
system during the re-changed measurement gap to obtain channel
information for the base station of the second communication
system.
12. A method for adaptively determining a measurement gap to
discover another communication system, comprising: receiving
measurement gap scheduling information having random
characteristics assigned from a base station; if a preamble signal
of another communication system is not received during the
measurement gap having the random characteristics, adaptively
changing the measurement gap; and if the preamble signal of another
communication system is received during the changed measurement
gap, re-changing the measurement gap to correspond to a
transmission period of the preamble signal.
Description
TECHNICAL FIELD
[0001] The present invention relates to a mobile communication
system, and more particularly, to a method for receiving a preamble
signal from other communication system and a method for adaptively
changing a measurement gap to discover other communication
system.
BACKGROUND ART
[0002] A worldwide interoperability for microwave access (WiMAX)
system has a variety of frame lengths of 2, 5, 10, and 20 ms. When
a minimum frame size is assigned for a measurement gap to discover
a preamble of a frame start position, the opportunity to discover
the preamble is given at least once. However, in the WiMAX system
having a frame size of 20 ms, if a mobile station of a long term
evolution (LTE) system successively consumes 20 ms or more in order
to discover an adjacent WiMAX base station, the mobile station may
encounter a serious service delay. Accordingly, a method of
reducing the size of one measurement gap and assigning the
measurement gap at regular patterns or randomly has been
proposed.
[0003] FIG. 1 is a view illustrating a process for discovering a
preamble of an adjacent WiMAX base station according to a
measurement gap size while a mobile station of an LTE system
provides an evolved universal terrestrial radio access network
(E-UTRAN) service.
[0004] Referring to FIG. 1, {circle around (a)} shows a case where
one measurement gap size is 5 ms and half of one radio frame of 10
ms is assigned for a measurement gap. The measurement gap of 5 ms
is randomly assigned within one radio frame and WiMAX preambles are
detected in the second and fourth measurement gaps. However, since
the third and fourth measurement gaps are successively assigned,
the mobile station can not receive an E-UTRAN service for a total
of 10 ms. Therefore, as shown in {circle around (b)} of FIG. 1, one
measurement gap size may be reduced to 2.5 ms and a measurement gap
may be randomly assigned within one radio frame. However, since a
time assigned for the measurement gap is less than the case of
{circle around (a)}, a time for discovering the WiMAX preamble may
be delayed.
[0005] When handover is performed in the LTE system, it is
necessary to determine a channel measurement interval to recognize
the existence of an adjacent base station and to report a channel
state. However, in the case where a candidate base station differs
from a currently connected base station in frequency or radio
access technology (RAT), there has been no way to allocate an
interval for discovering adjacent base stations and measuring a
channel state of corresponding base stations while minimizing a
service interruption time with the currently connected base
station.
DISCLOSURE
Technical Problem
[0006] An object of the present invention devised to solve the
problem lies in providing a method for receiving a preamble signal
of another communication system and a method for adaptively
changing a measurement gap to discover another communication
system.
Technical Solution
[0007] The object of the present invention can be achieved by
providing a method for receiving a preamble signal from a base
station of a second communication system by a communicating mobile
station in a first communication system. The method includes
receiving measurement gap scheduling information from a base
station of the first communication system, if a preamble signal of
the second communication system is not received during an assigned
measurement gap, changing the measurement gap, and receiving the
preamble signal of the second communication system during the
changed measurement gap.
[0008] In another aspect of the present invention, provided herein
is a method for receiving a preamble from a base station of a
second communication system by a communicating mobile station in a
first communication system, including, if a preamble signal of the
second communication system is not received during a measurement
gap assigned from a base station of the first communication system,
adaptively changing the measurement gap, receiving the preamble
signal of the second communication system during the changed
measurement gap and re-changing the changed measurement gap to
correspond to a transmission period of the preamble signal, and
further receiving the preamble signal of the second communication
system during the re-changed measurement gap to obtain channel
information for the base station of the second communication
system.
[0009] In a further aspect of the present invention, provided
herein is a method for adaptively determining a measurement gap to
discover another communication system, including receiving
measurement gap scheduling information having random
characteristics assigned from a base station, if a preamble signal
of another communication system is not received during the
measurement gap having the random characteristics, adaptively
changing the measurement gap, and if the preamble signal of another
communication system is received during the changed measurement
gap, re-changing the measurement gap to correspond to a
transmission period of the preamble signal.
[0010] In the above aspects of the present invention, no signal may
be transmitted and received to and from the base station of the
first communication system during the measurement gap.
[0011] The measurement gap scheduling information may include a
measurement gap size and a measurement gap assignment element. The
changing the measurement gap may change at least one of the
measurement gap size and the measurement gap assignment
element.
[0012] The measurement gap assignment element may include at least
one of a measurement gap interval, a measurement window, and the
number of times measurement is performed during the measurement
window.
[0013] The measurement gap scheduling information may be index
information based on a measurement gap pattern table in which
measurement gap scheduling parameter values are set.
[0014] The changed measurement gap after the preamble signal of the
second communication system is received may be re-changed to
correspond to a preamble transmission period of the second
communication system.
[0015] In the above first aspect of the present invention, the
method may further includes transmitting base station
identification information obtained from the preamble signal of the
second communication system to the base station of the first
communication system, and receiving system information for the
second communication system from the base station of the first
communication system.
[0016] In the above three aspects of the present invention, the
mobile station may further receive the preamble signal of the
second communication system at least once to obtain channel quality
information for the second communication system. Further, if a
value of the obtained channel quality information for the second
communication system is above a prescribed reference, the mobile
station may perform handover to the base station of the second
communication system.
ADVANTAGEOUS EFFECTS
[0017] The present invention provides a scheduling method of a
measurement gap for discovering other systems to a mobile station,
thereby effectively assuring mobility of the mobile station.
Moreover, the measurement gap is efficiently re-scheduled to raise
the probability of detecting a preamble signal of another system.
In addition, a time for the mobile station to discover another
system can be minimized.
DESCRIPTION OF DRAWINGS
[0018] The accompanying drawings, which are included to provide a
further understanding of the invention, illustrate embodiments of
the invention and together with the description serve to explain
the principle of the invention.
[0019] In the drawings:
[0020] FIG. 1 is a view illustrating a process for discovering a
preamble of an adjacent WiMAX base station according to measurement
gap sizes while a mobile station of an LTE system provides an
E-UTRAN service;
[0021] FIG. 2 is a flow chart illustrating a method for discovering
another communication system according to an exemplary embodiment
of the present invention;
[0022] FIG. 3 is a view illustrating parameters used to determine a
measurement gap according to an exemplary embodiment of the present
invention;
[0023] FIG. 4 is a view illustrating a method for changing a
measurement gap according to an exemplary embodiment of the present
invention;
[0024] FIG. 5 is a view illustrating a method for changing a
measurement gap according to another exemplary embodiment of the
present invention;
[0025] FIG. 6 is a view illustrating a method for changing a
measurement gap according to still another exemplary embodiment of
the present invention;
[0026] FIG. 7 is a view illustrating a method for changing a
measurement gap according to a further exemplary embodiment of the
present invention;
[0027] FIG. 8 is a flow chart illustrating a method for a mobile
station to change a measurement gap according to an exemplary
embodiment of the present invention; and
[0028] FIG. 9 is a flow chart illustrating a method for a base
station to change a measurement gap according to an exemplary
embodiment of the present invention.
MODE FOR INVENTION
[0029] Reference will now be made in detail to the exemplary
embodiments of the present invention, examples of which are
illustrated in the accompanying drawings. The detailed description,
which will be given below with reference to the accompanying
drawings, is intended to explain exemplary embodiments of the
present invention, rather than to show the only embodiments that
can be implemented according to the invention. The following
detailed description includes specific details in order to provide
a thorough understanding of the present invention. However, it will
be apparent to those skilled in the art that the present invention
may be practiced without such specific details. For example, the
following description will be given centering around specific
terms, but the present invention is not limited thereto and any
other terms may be used to represent the same meanings.
[0030] In some instances, known structures and/or devices are
omitted or are shown in block diagram and/or flow chart form,
focusing on important features of the structures and/or devices, so
as not to obscure the concept of the present invention. The same
reference numbers will be used throughout this specification to
refer to the same or like parts.
[0031] Exemplary embodiments described herein below are
combinations of elements and features of the present invention. The
elements or features may be considered selective unless otherwise
mentioned. Each element or feature may be practiced without being
combined with other elements or features. Further, an embodiment of
the present invention may be constructed by combining parts of the
elements and/or features. Operation orders described in embodiments
of the present invention may be rearranged. Some constructions of
any one embodiment may be included in another embodiment and may be
replaced with corresponding constructions of another
embodiment.
[0032] In exemplary embodiments of the present invention, a
description is made of a data transmission and reception
relationship between a base station and a mobile station. Here, the
term `base station` refers to a terminal node of a network
communicating directly with the mobile station. In some cases, a
specific operation described as performed by the base station may
be performed by an upper node of the base station. Namely, it is
apparent that, in a network comprised of a plurality of network
nodes including a base station, various operations performed for
communication with a mobile station may be performed by the base
station, or other network nodes except for the base station. The
term `base station` may be replaced with the term `fixed station`,
`Node B`, `eNode B` (eNB), `access point`, etc. The term `mobile
station` may be replaced with the term `mobile terminal`, `user
equipment` (UE), `mobile subscriber station` (MSS), etc.
[0033] FIG. 2 is a flow chart illustrating a method for discovering
another communication system according to an exemplary embodiment
of the present invention.
[0034] A method for a mobile station which is communicating with a
base station #1 in a first communication system to discover a
second base station #2 of a second communication system which is an
adjacent base station will be described with reference to FIG.
2.
[0035] A method for recognizing a corresponding communication
system by receiving a preamble signal transmitted periodically by a
base station of another communication system may be an example of a
method for discovering another communication system. The preamble
is a signal which is transmitted first in a frame transmitted by
the base station and has a regular pattern. Therefore, a mobile
station receiving the preamble can recognize the base station
transmitting the preamble. The mobile station can also estimate
timing for an initial symbol, detect the frame, and adjust time and
frequency synchronization, using the periodically transmitted
preamble signal.
[0036] First, a base station #1 assigns a measurement gap for a
mobile station in step 10. The measurement gap may be assigned by a
request of the mobile station or by decision of the base station.
The base station #1 transmits scheduling information as to
measurement gap assignment, that is, measurement gap scheduling
information to the mobile station in step 11. The mobile station
receives the information about the measurement gap from the base
station #1 and starts measurement in step 12. Namely, the mobile
station monitors whether a preamble is detected which is
transmitted by other base stations during an assigned measurement
gap from frames started after the measurement gap scheduling
information is received.
[0037] If the mobile station does not discover another base station
during the measurement gap assigned by the base station for a
prescribed time period, the mobile station changes the measurement
gap in step 13. Various methods may be applied to change the
measurement gap according to scheduling parameters determining the
measurement gap. It is desirable to change the measurement gap if
the probability of discovering another base station is higher than
the probability when using the measurement gap assigned by the base
station.
[0038] A process of changing the measurement gap may be performed
by receiving the measurement gap scheduling information, which is
scheduled by the base station and changed by the base station.
Alternatively, the process of changing the measurement gap may be
performed by the mobile station itself without additional
scheduling information or any direction from the base station. If
the mobile station changes the measurement gap, it is desirable to
share the information about the changed measurement gap with the
base station.
[0039] Although FIG. 1 shows one measurement gap changing process,
it is apparent that the measurement gap changing process can be
performed once or more if another base station is not discovered
for a prescribed time period even during the changed measurement
gap.
[0040] Through a process of increasing the probability of receiving
the preamble of another base station by the measurement gap
changing process, the mobile station receives the preamble from
another base station, that is, from the base station #2 of the
second communication system.
[0041] Using the foregoing-described method for receiving the
preamble of another communication system, the preamble may be more
efficiently received when a preamble transmission period of the
base station of another communication system is not known.
Hereinafter, a method of efficiently receiving a preamble using
information about the preamble transmission period of the base
station will be described.
[0042] Identification (ID) information about the base station #2
can be acquired through the preamble of the base station #2.
Assuming that a cell ID is assigned to each base station, base
station ID information may be a cell ID.
[0043] The mobile station acquires the base station ID information
through the received preamble and transmits the ID information of
the base station #2 to the base station #1 in step 15. If the ID
information of the base station #2 is transmitted to the base
station #1, information as to a preamble transmission period of the
base station #2 which has transmitted the preamble can be acquired
from the base station #1. The information as to the preamble
transmission period may be a frame size for example. Upon receiving
the base station ID information from the mobile station, the base
station #1 transmits information about the frame size of a
corresponding base station to the mobile station in step 16.
[0044] In the case where the base station #1 already knows the
frame size of the base station #2, the base station #1 can directly
inform the mobile station of the frame size without an additional
procedure if the base station #1 receives the base station ID
information from the mobile station. However, if the base station
#1 does not know information about the base station #2, the base
station #1 obtains information about the frame size through
communication with an entity, which might know system information
of the base station #2, for example, an inter-working gateway, and
informs the mobile station of the frame size of the base station
#2.
[0045] The mobile station which has received information about the
frame size of the base station #2 from the base station #1
re-changes the measurement gap based on the received frame size.
That is, if the frame size of the base station #2 is known, since a
preamble is transmitted at a transmission start part of each frame,
the preamble transmission period of the base station #2 can be
known. Accordingly, if the measurement gap is re-changed according
to the frame size, the base station #2 can further receive the
preamble without wasting resources because the base station #2
performs measurement in synchronization with preamble transmission
timing.
[0046] If the mobile station desires to perform handover to the
discovered base station, the base station #2 for example, the
mobile station should confirm channel information about the base
station #2. It is difficult to recognize the channel information by
one received preamble and the channel information can be acquired
by receiving a plurality of preambles.
[0047] The mobile station receives one or more preambles during a
measurement gap synchronized with the preamble transmission period
of the base station #2 in step 18 and obtains channel quality
information about the base station #2 in step 19. If the obtained
channel quality value is above a prescribed threshold value, the
mobile station may start handover to the base station #2.
[0048] FIG. 3 is a view illustrating parameters used to determine a
measurement gap according to an exemplary embodiment of the present
invention.
[0049] To start measurement, a mobile station which is
communicating with a base station should know a measurement start
reference time point. After the reference time point, the mobile
station performs measurement at a regular or irregular time
interval according to scheduling information of the base station.
An interval until the next measurement is started may be regular or
irregular according to scheduling information.
[0050] The measurement gap scheduling information includes not only
information about a time point at which measurement is started but
also information about the size of one measurement gap and as to
how a measurement gap is assigned. The above information may be
referred to as a measurement start point, a measurement gap size,
and a measurement gap assignment scheme. The measurement gap
scheduling information is provided by the base station to the
mobile station periodically or through control signaling.
[0051] The measurement start point is a parameter representing a
time point indicating that the mobile station which is
communicating with the base station starts measurement for another
base station. The measurement start point may be determined based
on a subframe number as an initial frame number. After this
reference time point, the mobile station starts measurement
according to the measurement gap scheduling information received
from the base station. The measurement gap size is the size of one
measurement gap and may be determined as a relative value for one
frame size.
[0052] The measurement gap assignment scheme for assigning a
measurement gap is classified into a periodic method and an
aperiodic method. When the measurement gap is assigned to
periodically perform measurement, a measurement gap interval, and a
measurement window which is an interval during which measurement is
performed may be elements for assigning the measurement gap.
[0053] When the measurement gap is assigned to aperiodically
perform measurement, the measurement window, and the number of
times measurement is performed per window, that is, a measurement
frequency, may be elements for assigning the measurement gap. The
measurement window may be determined by the number of frames in
which measurement is performed.
[0054] As mentioned above, during measurement gap scheduling, the
base station determines the above-described parameters and informs
the mobile station of the parameters as the measurement gap
scheduling information.
[0055] The base station may directly inform the mobile station of
each parameter value of the measurement gap scheduling information.
Alternatively, the base station may have a measurement gap pattern
table shared with the mobile station and may inform the mobile
station of index information based on the measurement gap pattern
table. Since the amount of scheduling information can be reduced
using the index information based on this table, communication
efficiency is increased and resources can be effectively used.
[0056] The base station sets each parameter value of the
measurement gap scheduling information in the measurement gap
pattern table and informs the mobile station of measurement start
point information and determined index information when
transmitting initial measurement gap scheduling information. The
mobile station then performs measurement during an assigned
measurement gap according to the received measurement start point
and index information.
[0057] Table 1 illustrates an example of the measurement gap
pattern table.
TABLE-US-00001 TABLE 1 Measurement Measurement Measurement
Measurement Gap Interval Window Frequency Index Gap Size (Frame)
(Frame) (/window) 1 1 5 10 N/A 2 1 3 10 N/A 3 1 Random 8 3 4 2
Random 8 4 5 2 Random 4 4
[0058] Assuming that the base station transmits information
indicating index 2 of the measurement gap pattern table together
with measurement start point information to inform the mobile
station of measurement gap scheduling information, the mobile
station periodically performs measurement with a measurement gap
interval of 3 frames during a measurement gap having a size of 1
within a measurement window of 10 frames from a measurement start
point as indicated in Table 1.
[0059] If the base station transmits information indicating index 4
of the measurement gap pattern table together with measurement
start point information to inform the mobile station of measurement
gap scheduling information, the mobile station randomly performs
measurement four times during a measurement gap having a size of 2
within a measurement window of 8 frames from a measurement start
point as indicated in Table 1. The random assignment may be
performed by the mobile station and reported to the base station.
Conversely, the random assignment may be performed by the base
station and reported to the mobile station. In Table 1, if the
measurement gap interval is random, it is desirable that the base
station know the randomly assigned measurement gap through
information exchange with the mobile station.
[0060] Since the base station informs the mobile station of the
index indicating measurement gap scheduling information of a
specific pattern, the mobile station can know the measurement gap
scheduling information and flexibly change a measurement gap.
[0061] Hereinafter, a method for changing the measurement gap using
the above-described measurement gap scheduling information will be
described in detail. Especially, the following exemplary
embodiments describe the case where a mobile station which is
communicating in an LTE system desires to perform handover to a
base station of a WiMAX system. The configuration of a radio
interface of the mobile station may be applied irrespective of one
radio interface or dual radio interfaces.
[0062] A base station of an LTE system will be referred to as an
E-nodeB and a base station of a WiMAX system will be referred to as
a WiMAX BS.
[0063] When the mobile station performs handover from the LTE
system to the WiMAX system or from the WiMAX system to the LTE
system, an E-UTRAN based mobile station should be able to discover
an adjacent WiMAX system before attempting to perform the handover.
To this end, the base station schedules a scanning interval, that
is, a measurement gap to discover the WiMAX system for a prescribed
time.
[0064] FIG. 4 is a view illustrating a method for changing a
measurement gap according to an exemplary embodiment of the present
invention.
[0065] In the method for changing a measurement gap according to
the embodiment of the present invention, if a WiMAX BS is not found
during an initial setting window, measurement gap scheduling
parameters are changed to decrease the size of an initially set
measurement window and to maintain a measurement frequency per
window.
[0066] Assuming that WiMAX BSs having frame sizes of 5, 10, and 20
ms are present, preambles are periodically transmitted as shown in
FIG. 4.
[0067] In FIG. 4, {circle around (a)} shows a measurement method
according to an initially set measurement gap. In the initially set
measurement gap, a measurement window size is 4 and a measurement
frequency is 3. That is, measurement is randomly performed 3 times
during 4 frames. If the WiMAX BS is not discovered during the
initially set measurement gap, the measurement scheduling
parameters are changed.
[0068] In FIG. 4, {circle around (b)} shows a measurement method
according to a changed measurement gap. In the changed measurement
gap, a measurement window size is 2, which is half the initially
set window size, and a measurement frequency is 3, which is the
same as the initially set measurement frequency. That is,
measurement is randomly performed 3 times during 2 frames.
Therefore, a measurement gap interval is substantially reduced as
compared to {circle around (a)}, thereby increasing the probability
of discovering an adjacent base station. If the WiMAX BS is not
discovered even during the changed measurement gap, the measurement
scheduling parameters can be changed again.
[0069] In FIG. 4, {circle around (c)} shows a measurement method
according to a re-changed measurement gap. In the re-changed
measurement gap, a measurement window size is 1, which is half the
set window size in {circle around (b)}, and a measurement frequency
is 3, which is the same as the initially set measurement frequency.
Namely, measurement is randomly performed 3 times during one frame.
This method substantially reduces the measurement gap interval as
compared to {circle around (a)} and {circle around (b)}, thereby
increasing the probability of discovering the WiMAX BS.
[0070] FIG. 5 is a view illustrating a method for changing a
measurement gap according to another exemplary embodiment of the
present invention.
[0071] In the method for changing a measurement gap, if a WiMAX BS
is not found during an initially set window, measurement gap
scheduling parameters are changed to increase the measurement
frequency per window while maintaining the size of the initially
set window.
[0072] Assuming that WiMAX BSs having frame sizes of 5, 10, and 20
ms are present, preambles are periodically transmitted as shown in
FIG. 5.
[0073] In FIG. 5, {circle around (a)} shows a measurement method
according to an initially set measurement gap. According to the
initially set measurement gap, a measurement window size is 4 and a
measurement frequency is 2. That is, measurement is randomly
performed twice during 4 frames. If the WiMAX BS is not discovered
during the initially set measurement gap, the measurement
scheduling parameters are changed.
[0074] In FIG. 5, {circle around (b)} shows a measurement method
according to a changed measurement gap. In the changed measurement
gap, a measurement window size is 4, which is the same as the
initially set window size, and a measurement frequency is 4, which
is twice the initially set measurement frequency. That is,
measurement is randomly performed 4 times during 4 frames.
Therefore, a measurement gap interval is substantially reduced as
compared to {circle around (a)}, thereby increasing the probability
of discovering an adjacent base station. If the WiMAX BS is not
discovered even during the changed measurement gap, the measurement
scheduling parameters can be changed again.
[0075] In FIG. 5, {circle around (c)} shows a measurement method
according to a re-changed measurement gap. In the re-changed
measurement gap, a measurement window size is 4, which is the same
as the initially set window size, and a measurement frequency is 6,
which is increase by 2 from the measurement frequency set in
{circle around (b)}. That is, measurement is randomly performed 6
times during 4 frames. This method substantially reduces the
measurement gap interval as compared to {circle around (a)} and
{circle around (b)} of FIG. 5, thereby increasing the probability
of discovering the WiMAX BS.
[0076] In this exemplary embodiment, since the initially set
measurement window value is not changed, a time point at which the
measurement gap is configured by successive frames can be delayed
compared with the exemplary embodiment of FIG. 4 and a service
interruption time from the base station can be reduced.
[0077] The method for changing the measurement gap described with
reference to FIGS. 4 and 5 may be repeatedly performed until an
adjacent base station is discovered. Although the measurement gap
may be changed by scheduling by the base station, the mobile
station may actively re-schedule the measurement gap after
receiving the initial measurement gap scheduling information from
the base station, thereby increasing the probability of discovering
an adjacent base station. In this case, a basic measurement gap
size may be kept constant or may be changed. If scheduling is
changed by the mobile station, it is desirable to report such
information to the base station.
[0078] Through the above-described method for changing the
measurement gap, the mobile station can efficiently perform
discovery of an adjacent base station. Hereinafter, a method for
efficiently receiving a preamble after the mobile station discovers
an adjacent base station, that is, after a preamble of the adjacent
base station is initially received will be described in detail.
[0079] If a preamble transmission period of an adjacent base
station is not known, the measurement gap is randomly assigned.
However, if the preamble transmission period of the adjacent base
station is recognized, the measurement gap may be periodically
assigned according to the transmission period to receive one or
more preambles without wasting resources.
[0080] FIG. 6 is a view illustrating a method for changing a
measurement gap according to still another exemplary embodiment of
the present invention.
[0081] This exemplary embodiment is applied when a communicating
base station knows a preamble transmission period of an adjacent
base station discovered by a mobile station. It is also assumed
that a method for increasing a measurement frequency while
maintaining a measurement window size is applied as a measurement
gap changing method.
[0082] Referring to FIG. 6, an example is shown of a method for
determining a window-based measurement gap when a mobile station of
an LTE system performs handover to a WiMAX system from the LTE
system. Assuming that a WiMAX BS has a frame size of 5 ms,
preambles are periodically transmitted as shown in FIG. 6.
[0083] In FIG. 6, {circle around (a)} shows a measurement method
according to an initially set measurement gap. According to the
initially set measurement gap, a measurement window size is 4 and a
measurement frequency is 2. That is, measurement is randomly
performed twice during 4 frames. If the WiMAX BS is not discovered
during the initially set measurement gap, measurement scheduling
parameters are changed.
[0084] In FIG. 6, {circle around (b)} shows a measurement method
according to a changed measurement gap. In the changed measurement
gap, a measurement window size is 4, which is the same as the
initially set window size, and a measurement frequency is 4, which
is increased by 2 from the initially set measurement frequency.
That is, measurement is randomly performed 4 times during 4 frames.
In this case, a preamble of the WiMAX system is detected at the
third measurement gap.
[0085] The mobile station obtains information capable of
identifying a WiMAX BS, a cell ID for example, from a sequence of
the received preamble and transmits the cell ID to a currently
communicating E-nodeB. Since the E-nodeB already knows system
information of the WiMAX BS, the E-nodeB confirms the WiMAX BS
using the received cell ID and notifies the mobile station of cell
information including a frame size for the confirmed WiMAX BS. The
cell information may include system information.
[0086] The mobile station may re-change a measurement gap using the
frame size received from the E-nodeB. To further receive the
preamble of the WiMAX BS, the mobile station changes the
measurement gap according to a preamble transmission period. Since
the preamble transmission period has a value corresponding to a
frame size when considering that the preamble is transmitted at the
first part of each frame, the preamble can be efficiently received
by re-changing the measurement gap using the frame size.
[0087] As another method, the E-nodeB receiving the cell ID can
assign a measurement gap after an offset corresponding to a WiMAX
frame size from a time the cell ID is received at. In this case, it
is desirable to determine a measurement gap interval using the
WiMAX frame size.
[0088] To periodically further receive the preamble of the WiMAX BS
after the WiMAX frame size since the first preamble has been
detected, the mobile station further receives at least one preamble
during a periodically assigned measurement gap, thereby measuring a
channel station through a downlink signal.
[0089] If the E-nodeB does not have frame size information of the
WiMAX system for the cell ID transmitted by the mobile station, the
E-nodeB may acquire corresponding information through the WiMAX BS
or a middle entity.
[0090] FIG. 7 is a view illustrating a method for changing a
measurement gap according to a further exemplary embodiment of the
present invention.
[0091] This exemplary embodiment is applied when a communicating
base station does not sufficiently know system information about an
adjacent base station of another communication system discovered by
the mobile station. This exemplary embodiment shows a procedure
performed by a mobile station to discover a preamble of an adjacent
base station, transmit a cell ID of another communication system to
the base station, and wait for the reception of additional
information.
[0092] An E-nodeB may request that an adjacent WiMAX BS transmit
additional information such as downlink channel descriptor (DCD)
and uplink channel descriptor (UCD) within the next few frames for
the mobile station which may perform handover to the WiMAX BS.
[0093] Referring to FIG. 7, an example is shown of a method for
determining a window-based measurement gap when a mobile station of
an LTE system performs handover to a WiMAX system from the LTE
system. Assuming that the WiMAX BS has a frame size of 5 ms,
preambles are periodically transmitted as shown in FIG. 7. It is
assumed that a measurement gap changing method for reducing a
measurement window size and maintaining a measurement frequency is
applied.
[0094] In FIG. 7, {circle around (a)} shows a measurement method
according to an initially set measurement gap. In the initially set
measurement gap, a measurement window size is 4 and a measurement
frequency is 3. That is, measurement is randomly performed 3 times
during 4 frames. If the WiMAX BS is not discovered during the
initially set measurement gap, measurement scheduling parameters
are changed.
[0095] In FIG. 7, {circle around (b)} shows a measurement method
according to a changed measurement gap. According to the changed
measurement gap, a measurement window size is 2, which is half the
initially set window size, and a measurement frequency is 3, which
is the same as the initially set measurement frequency. That is,
measurement is randomly performed 3 times during 2 frames. If the
WiMAX BS is not discovered even during the changed measurement gap,
the measurement scheduling parameters can be changed again.
[0096] In FIG. 7, {circle around (c)} shows a measurement method
according to a re-changed measurement gap. In the re-changed
measurement gap, a measurement window size is 1, which is half the
changed set window size of {circle around (b)}, and a measurement
frequency is 3, which is the same as the initially set measurement
frequency. Namely, measurement is randomly performed 3 times during
one frame. In this case, a preamble of the WiMAX system is received
at the second measurement gap.
[0097] The mobile station obtains information capable of
identifying the WiMAX BS, a cell ID for example, through a sequence
of the received preamble and transmits the cell ID to the E-nodeB
which is the base station of the communicating LTE system. The
E-nodeB informs the mobile station of the frame size of the WiMAX
system through the received cell ID.
[0098] If the E-nodeB does not have frame size information of the
WiMAX system for the cell ID transmitted by the mobile station, the
E-nodeB may acquire corresponding information through the WiMAX BS
or a middle entity. When the WiMAX BS transmits system information
through communication with an adjacent WiMAX BS, the E-nodeB may
request that the WiMAX BS transmit additional information such as
DCD and UCD. Therefore, when the mobile station detects an
additional preamble, the information about the DCD and UCD can be
previously acquired.
[0099] The mobile station can re-change a measurement gap through
the frame size information received from the E-nodeB. The mobile
station adjusts the measurement gap to a preamble transmission
period to further receive the preamble of the WiMAX BS. Since the
preamble transmission period becomes a value corresponding to a
frame size when considering that the preamble is transmitted at the
first part of each frame, if the measurement gap is re-changed
through the frame size, the preamble can be efficiently
received.
[0100] As another method, the E-nodeB receiving the cell ID may
assign a measurement gap after an offset corresponding to a WiMAX
frame size from a time the cell ID is received at. In this case, it
is desirable to determine a measurement gap interval by the WiMAX
frame size.
[0101] To periodically further receive the preamble of the WiMAX BS
after the WiMAX frame size since the first preamble has been
detected, the mobile station further receives at least one preamble
through a periodically assigned measurement gap, thereby measuring
a channel station through a downlink signal.
[0102] FIG. 8 is a flow chart illustrating a method for a mobile
station to change a measurement gap according to an exemplary
embodiment of the present invention.
[0103] Referring to FIG. 8, a method for a communicating mobile
station in an LTE system which has successfully received a preamble
of a WiMAX BS to obtain information necessary for handover by
periodically changing a measurement gap is shown. In FIG. 8, `A`
and `B` refer to frames along a time axis of the mobile station
having one radio interface in a WiMAX system and an LTE system,
respectively.
[0104] The mobile station (or UE) receives a preamble of the WiMAX
system in step 81. The mobile station acquires a cell ID of the
WiMAX system through sequence information of the received preamble
and stores a frame number of the WiMAX system, a preamble of which
has been detected.
[0105] The mobile station transmits the cell ID and the frame
number to an E-nodeB in step 82 to inform the E-nodeB that an
adjacent WiMAX system has been discovered. The E-nodeB which has
received the cell ID and the frame number from the mobile station
requests a WiMAX frame size corresponding to a corresponding cell
through communication with the WiMAX BS or a middle gateway for
inter-working between RATs in step 83.
[0106] If the E-nodeB desires to inform the mobile station of
additional information about an adjacent WiMAX BS, the E-nodeB may
request that the WiMAX BS transmit the additional information. In
this case, the E-nodeB transmits the frame number received from the
mobile station so that the adjacent WiMAX BS can inform the E-nodeB
of corresponding additional information after the frame number.
[0107] The additional information may be cell information including
system information such as DCD and UCD. The requested additional
information may be transmission intervals of the DCD and UCD, frame
numbers of the DCD and UCD transmitted after the received frame
number, and the like.
[0108] In step 84, the WiMAX BS transmits a response to the request
in step 83. The response may include transmission intervals of the
DCD and UCD and frame numbers of the DCD and UCD transmitted after
the received frame number, as well as a frame size. In step 85, the
E-nodeB transmits WiMAX system information received in step 84 to
the mobile station.
[0109] The mobile station can recognize a preamble transmission
point of the WiMAX system by the frame number stored in step 81 and
an offset corresponding to the frame size received in step 85, and
can estimate a frame number at which the UCD and DCD information is
to be transmitted. Therefore, the mobile station changes a
measurement gap using the frame number stored in step 81 and
information received in step 85 to re-schedule the measurement gap
in step 86.
[0110] The mobile station transmits information about the
measurement gap changed in step 86 to the E-nodeB to request the
E-nodeB to assign the measurement gap in step 87. The E-nodeB
transmits a response to the request in step 87 to the mobile
station to inform the mobile station whether the measurement gap is
assigned in step 88.
[0111] If the measurement gap has been successfully assigned in
step 88, the mobile station can periodically receive preambles of
the WiMAX system every measurement gap interval requested in step
87 as indicated in steps 89, 890, and 891. Thus the mobile station
has the opportunity to measure channel quality for the WiMAX system
in step 892 by receiving a plurality of preambles of the WiMAX
system.
[0112] The mobile station may previously acquire DCD and UCD
information of the WiMAX system and have the opportunity of
reducing a delay when performing a registration procedure to the
WiMAX system.
[0113] The mobile station transmits a channel quality measurement
result for the WiMAX system to the base station in step 893. Then
the base station or the mobile station may determine whether to
perform handover to the WiMAX system of the mobile station based on
information about the channel quality measurement result.
[0114] FIG. 9 is a flow chart illustrating a method for a base
station to change a measurement gap.
[0115] Referring to FIG. 9, a method for a base station to obtain
information necessary for handover by periodically changing a
measurement gap with respect to a communicating mobile station in
an LTE system which has successfully received a preamble of a WiMAX
BS is shown. In FIG. 9, `A` and `B` refer to frames along a time
axis of the mobile station having one radio interface in a WiMAX
system and an LTE system, respectively.
[0116] A process of FIG. 9 is similar to a process of FIG. 8 but
the measurement gap is re-scheduled by the base station. Steps 91
to 94 are as described in FIG. 8. That is, the mobile station
receives a preamble of the WiMAX system, transmits a cell ID and a
frame number to the base station, and stores the cell ID and the
frame number. If the mobile station 82 informs the E-nodeB that a
preamble of the WiMAX system has been received, the E-nodeB
receives information about a frame size, transmission intervals of
DCD and UCD, and frame numbers of DCD and UCD to be transmitted
after the received frame number through communication with the
WiMAX BS.
[0117] In step 95, the base station re-schedules the measurement
gap by changing the measurement gap using the frame number and
information received in step 94. Then, in step 96, the E-nodeB
transmits information about the measurement gap changed in step 95
to the mobile station to notify the mobile station of the changed
measurement gap.
[0118] The mobile station receiving measurement gap scheduling
information changed in step 96 can periodically receive preambles
of the WiMAX system every measurement gap interval requested in
step 87 as indicated in steps 97, 98, and 99. Thus the mobile
station has the opportunity to measure channel quality for the
WiMAX system in step 990 by receiving a plurality of preambles of
the WiMAX system.
[0119] The mobile station may previously acquire DCD and UCD
information of the WiMAX system and have the opportunity of
reducing a delay when performing a registration procedure to the
WiMAX system.
[0120] The mobile station transmits a channel quality measurement
result for the WiMAX system to the base station in step 991. Then
the base station or the mobile station may determine whether to
perform handover to the WiMAX system of the mobile station based on
information about the channel quality measurement result.
[0121] The method for receiving the preamble signal of another
communication system and the method for changing the measurement
gap may be applied to an intra-frequency/RAT handover as well as an
inter-frequency/RAT handover.
[0122] It is apparent that the present invention may be embodied by
a combination of claims which do not have an explicit cited
relation in the appended claims or may include new claims by
amendment after application.
[0123] Embodiments of the present invention may be achieved by
various means, for example, hardware, firmware, software, or a
combination thereof. In a hardware configuration, a method for
receiving a preamble signal from other communication systems and a
method for adaptively changing a measurement gap according to
exemplary embodiments of the present invention may be achieved by
one or more application specific integrated circuits (ASICs),
digital signal processors (DSPs), digital signal processing devices
(DSPDs), programmable logic devices (PLDs), field programmable gate
arrays (FPGAs), processors, controllers, microcontrollers,
microprocessors, etc.
[0124] In a firmware or software configuration, a method for
receiving a preamble signal from a base station of another
communication system and a method for adaptively changing a
measurement gap according to exemplary embodiments of the present
invention may be achieved by a module, a procedure, a function,
etc. performing the above-described functions or operations.
Software code may be stored in a memory unit and executed by a
processor. The memory unit is located at the interior or exterior
of the processor and may transmit and receive data with the
processor via various known means.
[0125] It will be apparent to those skilled in the art that various
modifications and variations can be made in the present invention
without departing from the spirit or scope of the invention. Thus,
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *